A hydrodynamic bearing system essentially includes a bearing sleeve, a shaft accommodated in cylindrical inner bore of the bearing sleeve and at least one radial bearing section provided between the bearing sleeve and the shaft with the aid of which the shaft and the bearing sleeve are supported rotatably with respect to each other. A bearing gap formed between the shaft and the bearing sleeve is filled with a liquid lubricant, preferably bearing oil.
A hydrodynamic bearing system is also provided with a hydrodynamic thrust bearing to take on axial loads. Such thrust bearing is essentially formed by a thrust plate preferably arranged at one end of the shaft and a corresponding cover plate. The cover plate forms the counter bearing to the thrust plate and seals the entire bearing system from below so that no lubricant can escape from the bearing system.
As the motor starts up, the hydrodynamic pressure first builds up in the bearing system and then reaches its desired setpoint when the motor attaines its critical speed. Conversely, the hydrodynamic pressure in the bearing system reduces as the motor shuts down from its setpoint to zero when the motor comes to a standstill. During the transition phase described above, the hydrodynamic bearing system, and particularly the thrust bearing, does not perform its intended function which is due in part to the bearing surface of the thrust plate touching the bearing surface of the corresponding counter bearing. This results in increased wearing of these bearing surfaces and decreases the lifespan of the motor when there are frequent start-up/shut-down phases.